ABSTRACT
Knowing that animal life started in the sea, invertebrates have conquered the land from an aquatic habitat, a process called terrestrialization. This chapter explores the invertebrate terrestrializations using phylogenetic analysis. It starts the analysis by considering the fauna of pore water and groundwater. Among the pore water invertebrates, the chapter focuses on nematodes, tardigrades, and several groups of groundwater crustaceans. Animals that are completely restricted to life in groundwater are called stygobionts. The groundwater fauna shows a great similarity to the fauna of caves, in fact there are gradual transitions between cave habitats, karstic habitats, gravel habitats, and sediments. Collembola are also known from the fossil record. The first known hexapod, Rhyniella praecursor, is a collembolan from the early Devonian of the Rhynie Chert in Scotland, described in 1926. In the same deposition, slightly younger, remains have been found that were assigned to an ectognathic insect, Rhyniognatha.
